1. Field of the Invention
The present invention relates to the field of integrated circuit packaging, and more specifically to a low lead capacitance ceramic package for integrated circuits.
2. Related Art
Almost all semiconductor components are enclosed in packages when they are incorporated into a working electronic system. More specifically, integrated circuits are almost always enclosed in an appropriate package prior to their sale by the integrated circuit manufacturer.
There are many reasons for providing a proper encapsulation environment around an integrated circuit chip. The order of importance of these packaging factors depends on the particular component considered, and how it is to be tested and utilized. However, there are several functions of packaging for such chips which are common to all chips and environments. For example, appropriate packaging of an integrated circuit chip generally results in a dramatic economy in assembly. Integrated circuit chips are extremely small and fragile and, therefore, are difficult to handle without the aid of magnifying and manipulating equipment. Placement of a chip into an integrated circuit package overcomes the difficulty of handling the chip by providing a relatively large and easily handled package configuration. A second important function fulfilled by integrated circuit packages is mechanical protection of the chips. Because integrated circuit chips are generally comprised of a fragile material, they must be protected from the possibility of scratches and chipping. A third essential function of an integrated circuit package is to shield the enclosed integrated circuit chip from the surrounding environment so as to preclude corrosion and other detrimental effects resulting from exposure to the normal environment.
One frequently utilized integrated circuit package is the ceramic dual in-line package ("CERDIP"). In a CERDIP the body of the package is constructed of an extremely hard ceramic material, and metal electrical leads, which extend from the body, are configured in two parallel rows which are in line with one another.
A second frequently utilized integrated circuit package is the "CERQUAD". As in the CERDIP, the body of the CERQUAD is constructed of a hard ceramic material. However, metal leads extending from the body are configured as two sets of two parallel rows perpendicular to one another, typically with the ceramic body in a square configuration.
CERDIPs and CERQUADs are generally comprised of two basic components: (1) a base having an embedded lead frame, and (2) a cap. In the production of these ceramic packages, a glass material is screened onto a ceramic base (or substrate) so as to cover the base, leaving uncovered only a small chip cavity which is generally in the center of the substrate. Next, the metallic lead frame is positioned in the glass material on the package base such that when the base is heated to an elevated temperature, the glass material becomes molten and flows around the lead frame. Thus, when the glass material has cooled, the lead frame is embedded in the glass material, just above the package base.
The package cap is manufactured in essentially the same way as the package base, but without the embedding of a lead frame. In other words, the ceramic material which will become the package cap is screened with a glass material so as to cover all but a central area in the cap. The cap and glass material are then fired in a furnace to allow the glass material to adhere to the package cap material.
An integrated circuit manufacturer will use a package base and package cap in the following manner: An integrated circuit chip is placed in the central chip cavity in the package base. The integrated circuit chip is affixed to this central cavity, and is electrically connected, via minute wires, to the portion of the lead frame which extends around the central cavity. Once the integrated circuit chip is electrically and mechanically affixed to the package base, the cap is placed on the base and the entire unit is fired at a temperature higher than the temperature used to adhere the glass material to the package base and cap. During this last firing, the glass material may undergo a devitrifying process so as to render the glass material similar to a crystalline ceramic. This crystalline glass, having a greatly increased mechanical strength, then securely couples and seals the cap to the base, resulting in a single device. In most cases, however, a vitreous, nondevitrifying glass is used, which has mechanical strength equivalent to the older, devitrifying glass.
A great deal of research has been performed to develop glass materials which can be accurately screened onto ceramic materials, fired at a relatively low temperature to permit affixing of the glass material to the ceramic material and then fired at higher temperatures to make the final seal.
The development of a relatively low firing temperature solder glass has greatly contributed to the higher reliability of certain integrated circuits. However, this increase in reliability was accompanied by a corresponding and substantial detriment, in that to alter the chemical composition of the glass material, it was necessary to introduce materials such as zirconium and titanium. The introduction of such materials into the glass has significantly increased the dielectric constant in the glass, resulting in a significant increase in the parasitic capacitance between the closely aligned leads of the lead frame. This high parasitic lead capacitance has, in certain situations, precluded the utilization of low temperature ceramic packages for certain integrated circuit chips. Also, even if some integrated circuit chips can be successfully utilized in a low temperature ceramic package at a relatively low frequency, they cannot be used at higher frequencies because excessive parasitic capacitance results in undesirable attenuation, distortion, and cross talk of the electrical signals transmitted by the leads of the package.